Make path separator configurable for dir prompt
Make path separator configurable for dir prompt

Now the path separator can be configured by setting
`POWERLEVEL9K_DIR_PATH_SEPARATOR`

bhilburn/powerlevel9k,shibumi/powerlevel9k,dritter/powerlevel9k,rjorgenson/powerlevel9k,shibumi/powerlevel9k,dritter/powerlevel9k,natemccurdy/powerlevel9k,rjorgenson/powerlevel9k,bhilburn/powerlevel9k,natemccurdy/powerlevel9k